1. Field of the Invention
The invention is related to processes for producing metal powders having an extremely low gas and residual element concentration and in particular a time-temperature vacuum process based on the melting and boiling temperatures of the residual elements to be removed.
2. Prior Art
For years, metallurgists have recognized the need for high purity metals and, in particular, in the field of powder metallurgy as applied to the aircraft and aerospace industries. Although various methods and techniques are presently available for making relatively pure metals, the processes for converting these metals to powder form introduce gaseous inclusions and other contaminants, called residual or tramp elements, which are detrimental to subsequent processes. Typical methods for producing high quality metal powders are disclosed by Akers in U.S. Pat. No. 3,975,184, by Schlienger in U.S. Pat. No. 3,963,812, Kondo et al. in U.S. Pat. No. 3,887,402 and by Holland in U.S. Pat. No. 4,018,633. However, in all of these processes, the amount of trapped gases and residual or tramp elements are either increased or unaffected.
The prior art teaches several methods for the removal of trapped gases. The most common process used in industry today is to heat the metal in a vacuum such as taught by Roberts in U.S. Pat. No. 3,954,458. This process is extensively used in the vacuum tube industry where residual gases entrapped in the glass and metal are released inside the vacuum tube over a period of time. The released gases raise the pressure inside the vacuum tube and adversely affect its operation. The removal of residual oxygen by heating in a reducing atmosphere such as hydrogen is taught by Matt et al. in U.S. Pat. No. 3,744,993 as well as by Kondo et al. in U.S. Pat. No. 3,887,402 and Precht in U.S. Pat. No. 3,945,863. Another method for removing dissolved, absorbed or otherwise occluded gases from platinum is taught by Klemen in U.S. Pat. No. 3,511,640. In his process Klemen discloses mixing the metal powder to be degassed with an inert metal oxide powder and heating to about 1000.degree. C. to 1800.degree. C. for a period of time to dissipate substantially all the gases. The platinum powder is subsequently recovered by dissolving the metal oxide in an acid solution.
Disclosed herein is a new and different method for the removal of gases and residual elements from metallic powders by a time-temperature vacuum process based on the melting and boiling temperatures of the gas or residual element to be removed.